Counter-rotating machinery such as rotary pumps typically include one or more rotatable members such as impellers or screws mounted with very close clearances inside a cavity of the pump casing on shafts driven by a common prime mover. During operation, the rotatable members are subjected to rotational drive and axial and radial thrust forces. These forces may be empirically described as a dynamic system having six degrees of freedom. These are radial position of the member with respect to mutually orthogonal X and Y axes, axial position of the member along its longitudinal or Z axis, tilting of the member with respect to the X or Y axes (elevation and azimuth of the shaft), and rotation of the member about its longitudinal axis. The efficiency of a rotary pump depends upon friction free rotation of the shafts and maintenance of very close tolerances between the rotatable members and the walls of the pump cavity. Friction free rotation is, in turn, dependent upon minimization of restraint upon rotation of a shaft about its longitudinal axis while maintenance of very close tolerances is dependent upon maximization of restraint upon (indexing) the first five degrees of freedom. Usually, various combinations of dynamic shaft seals and shaft contacting bearings such as glide rings, slip rings or ball and race type bearings are selected to satisfy both of these requirements.
Shaft contacting type bearings require lubrication. Lubricants, however, produce molecular vapors at high shaft rotation speeds, e.g., above 10,000 rpm, which degrade the purity of a fluid passing through a pump. Recently, various configurations of permanent and electromagnets have been proposed as lubricant free substitutes for shaft contacting bearings at speeds in excess of 10,000 rpm. Proposed magnetic bearing designs in the field of rotary pumps have been limited to turbo-molecular type rotary pumps where a differential pressure in the field is obtained by transferring kinetic energy from impellers. Differential pressures provided by turbo-molecular type pumps are therefore limited to about 15 psig. Moreover, the proposed designs make no provision for restraining axial thrust forces and therefore necessitate the simultaneous use of either ball bearings, knife-edge ring bearings or discrete magnetic repulsion elements at one or both shaft ends. Consequently, the proposed magnetic bearing designs have little utility in positive displacement type rotary pumps where the axial thrust forces may be large and continuous in magnitude. Furthermore, it is difficult to combine a motor for driving a pump with the proposed magnetic bearing designs because drive motors are usually magnetically discontinuous with rotation while magnetic suspensions are preferably made magnetically continuous to reduce eddy current losses.